1. Apply a distinct object style to all containers for placed WMF graphics.

2. Use the UI's OBJECT Find/Replace functionality to search and to access the containers.

The following script written in ExtendScript will search all graphics placed in the active document and applies an object style named "WMF" to all frames that contain WMF graphics.

Define your own object style named "WMF" before or let the script do it. Wheras if you are preparing the object style you have more control over its definition (apart of the name) before running the script. You can undo the script's action in one go.

app.scriptPreferences.userInteractionLevel = UserInteractionLevels.interactWithAll;
app.doScript
(
applyObjectStyleToWMFContainers,
ScriptLanguage.JAVASCRIPT,
[],
UndoModes.ENTIRE_SCRIPT,
"Apply object style to WMF containers | SCRIPT"
);
function applyObjectStyleToWMFContainers()
{
// If no document is open, do nothing:
if(app.documents.length == 0){return};
// The name of the object style we like to apply
// to the containers of all placed WMF graphics:
var nameOfObjectStyle = "WMF";
var doc = app.documents[0];
var wmfObjectStyle = doc.objectStyles.itemByName(nameOfObjectStyle);
// Get all graphics in the document:
var allGraphics = doc.allGraphics;
var allGraphicsCount = allGraphics.length;
// If the objcet style with name in variable nameOfObjectStyle is not found
// create the object style:
if(!wmfObjectStyle.isValid)
{
doc.objectStyles.add({name : nameOfObjectStyle});
};
// Apply the object style to every container of a WMF found in the document.
// NOTE: Alternatively one can apply the object style to the WMF itself,
// but InDesign's UI for OBJECT Find/Replace would not be able to find it.
for(var n=0;n<allGraphicsCount;n++)
{
if(allGraphics[n].imageTypeName == "Windows Metafile")
{
allGraphics[n].parent.appliedObjectStyle = wmfObjectStyle;
}
}
// Use UI functionality OBJECT find/replace to search for all frames with object style by its name.
};

To distinguish MathType WMFs from none-MathType WMFs would require a deeper analysis of the graphics contents.